Process for making a mechanical pulp from wood chips



March l2, 1968 H, w, H, JONES ET AL 3,372,879

` PROCESS FOR MAKING A MECHANICAL PULP FROM WOOD CHIPS Filed Aug. 18, 1966 2 Sheets-Sheet 1 M'i 739W, l u

PATENT AGENT March 12, 1968 H w H, JQNES ET AL 3,372,879

PROCESS FOR MAKING A MECHANICAL PULP FROM WOOD CHIPS 2 Sheets-Sheet 2 Filed Aug. 1S, 196e BY /ym ATILNT AGENT United States Patent O 3,372,879 PROCESS FOR MAKING A MECHANICAL PULP FRQM WOOD CHIPS Howard W. H. Jones and Donald E. Helieur, GrandMere,

Quebec, Canada, assignors to Consolidated Paper (Bahamas) Limited, Nassau, Bahamas Continuation-impart of application Ser. No. 170,110,

Jan. 31, 1962. This application Aug. 18, 1966, Ser.

Claims. (Cl. 2411-24) This application is a continuation-in-part of application Ser. No. 170,110 filed Jan. 31, 1962, now abandoned.

This invention relates to a procss for converting raw wood by mechanical means into a wood pnl-p, and in particular, it relates to a mechanical pulping process for converting raw uncooked chips into a wood pulp.

Wood pulps which are produced by a procss employing mechanical means only, that is which do not rely on the addition of chemicais in the process, are known as mechanical pulps. Water or substances occurring naturally in the wood lmay be used in a process for producing a mechanical pulp, but the addition of chemicals does not take place for the pulp in its unbieached form; however, for bleached grades of mechanical pulp, chemicals are added which do not essentially change the yield of the unbleached pulp, but which do make the pulp appear brighter and whiter. On the other hand, when chemicals are added to convert raw wood into a pulp, the resulting pulp is known as a chemical pulp. If a process for producing a pulp uses a combination of a mechanical means and a chemical means, the resulting pulp is referred to as a high yield pulp or a semi-chemical pulp or a mechanochemical pulp. For all these pulps the yield is less than for a pure mechanical pulp, the lowest yield is for the purely chemical pulp and the yield for the mechano-chemical pulp is close to the mechanical pulp yield. This invention pertains to a mechanical pulping process which converts raw wood in the form of chips into a mechanical pulp.

A known process for making a mechanical wood pulp forces whole logs against a large revolving grindstone. The pulp produced by this process is commonly referred to as groundwood. In general, groundwood is cheaper than other known types of wood pulp, and the process provides more pulp for a given weight of wood than other pulping processes. The groundwood pulps, however, do not always have the necessary properties to give a iinisned paper product of the required quality. For example, the groundwood pulp, or paper made from groundwood pulp, may not have satisfactory runnability on a paper machine or in a printing press, or other properties in the finished paper product may not be satisfactory. As a result, various percentages of chemical and/ or semi-chemical pulps are incorporated with the groundwood pulp to give the mixed-furnish or the finished product the required properties. Since chemical pulps are expensive,

every effort is made to keep the percentage of the chemical pulp content as low as possible. It will be apparent that any improvement in the properties of a mechanical pulp which reduces the chemical pulp content necessary for a given quality of paper, would be Very desirable.

In the past, refinements in the groundwood process have been made which improve the properties of the groundwood pulp, and this has resulted in a reduction of the chemical pulp content required for a given quality in the finished product. Intensive studies have been made of the operating variables in the groundwood process, and the ranges of groundwood pulp properties that Imay be achieved by the controlled variation of these variables are known. It has been found, very generally, that the more energy used or consumed in the grinding process,

3,372,879 Patented Mar. I2, 1968 ICC the stronger will be the resulting pulp. There are, however, limits to the increase in strength that can be obtained by increasing the energy used per ton of pulp processed. One limitation is that the drainage rate decreases fairly quickly as more energy per ton is used in the groundwood process. The energy input cannot be increased past a point where the drainage rate or freeness becomes too low for a paper machine to operate properly. Thus, in the conventional groundwood process there is a definite limitation to the strength that can be achieved.

In another form of groundwood process it is known to subject whole logs to a coarse grinding step, to screen the resulting wood material, to refine the screen rejected material in a disc mill, and to combine the ground pulp with the pulp from the disc mill. This provides a pulp with strength properties generally less than or of the same order as that of conventional groundwood pulp. With this process also there are limitations to the power that can be applied to or absorbed by the pulp and conse-` characteristics of the pulp produced are very similar to the characteristics of groundwood pulp. Consequently the process is said to produce groundwood from chips. The strength of the pulp produced by the groundwood from chips process is generally less than or of the same order as that of conventional groundwood pulp.

The present invention seeks to provide a process for making an improved mechanical pulp from conventional wood chips. With the process of this invention increased amounts of energy can be absorbed by the wood material with a resulting increase in strength. The characteristics.

or properties of the pulp produced are quite distinguish able from the aforementioned conventional groundwood pulp or from groundwood pulp from chips.

4It is therefore an object of this invention to provide an improved process for making a mechanical pulp from.

wood chips.

It is another object of this invention to provide a process for producing a mechanical pulp that has a greater strength than comparable prior art mechanical pulps.

Further objects and advantages of the invention will be apparent from the following description taken in conjunc tion with the accompanying drawings in which,

FIGURE l is a flow sheet in block form of an overall mechanical pulp process in accordance with one embodiment of the invention,

FIGURE 2 is a simplified side view, partly in section, of part of a disc reiiner that may be used in the process of the invention,

FIGURE 3 is a fragmentary plan view of the plates used in the disc retiner of FIGURE 2,

FIGURE 4 is a side view, partly in section, of separating apparatus that may be used in the process of the invention, and

FIGURES 5a and 5b are graphs showing examples of*` 3 secondary acceptable pulp and a reject material, and mixing said primary and secondary acceptable pulps.

Referring now to FIGURE l, which shows a flow diagram in block form of the pulping process in accordance with one embodiment of the invention, the designation number represents a chip bin containing raw uncooked wood chips. Wood chips from chip bin 1t) are conveyed to a refiner stage 11 comprising a disc refiner which will be described in more detail hereinafter. The wood chips in refiner stage 11 are subjected to a continuously recurring flexing action to berize the chips. The movement of the Wood material through the refiner in stage 11 is restricted to prolong the treatment and permit a required amount of energy to be expended on the material. The output from refiner stage 1l is screened in coarse screens 12 with the rejected material being returned to refiner stage 11 for further treatment and the accepted material being passed to fine screens 14. The rejected wood material from fine screens 14 is fed into a mixing tank 15 and the accepted material is passed to a centrifugal separator or cleaner stage 16. The separator stage 16 may comprise a centrifugal separator of the vortex pressure drop type to be described in more detail in connection with FIGURE 4. This type of cleaner or separator is known in the prior art for separating small amounts of undesirable Wood material and dirt from a wood material being processed. The acceptable pulp from separator stage 16 may be fed to a thickening stage 17 or similar final stage, and the non-acceptable wood material is transferred to mixing tank 15.

It is important that the acceptable pulp be removed after the first stage of refining so that it will not be overprocessed in subsequent refining stages. This separation of the pulp after the first stage permits the necessary amount of processing to be used in the first stage to get a maximum yield of a good strength pulp from the first stage, and it also permits a suitable amount of additional refining to be given to those fractions of wood material which require it.

From the mixing tank 15 the wood material is transferred to a centrifugal separator 20 where it is separated into two fractions which may be referred to as a coarse fraction and a less coarse fraction. The separator 2t) is similar to that used in separator stage 16, except that it is operated differently in order to classify the material into two major fractions. As will be described subsequently, the stage 20 separator is operated at hydraulic pressures lower and reject rates higher than those in the more conventional separator stages 16, 26 and 27. Each of these two fractions from stage 20 are then subjected to a second stage of refining. The coarse fraction goes through a drainage stage 21 to a refiner stage 22. The block representing refiner stage 22 is labelled Refiners, Stages 2C meaning that it is a refiner refining a second stage of wood material and the wood material is the coarse fraction. The refiner stage 22 may comprise a disc refiner conventional in all respects and used in a conventional manner.

The less coarse fraction goes through drainage stage 23 to a refiner stage 24. The label on the block representing refiner stage 24 means that it is a refiner, second stage refining, of the less coarse fraction. The refiner Stage 24 may comprise a disc refiner conventional in all respects, or it may be a refiner similar to that used in stages 11 and 22 in which a special disc is used in a manner subsequently to be described.

The outputs from refiner stages 22 and 24 are cornbined and passed through fine screens 25. rIhe rejected wood material from fine screens 25 is returned to drainer stage 21 to become mixed with the coarse fraction from separator 20 and thereby reprocessed through refiner stage 22. The accepted wood material from fine screens 25 goes to a primary centrifugal separator stage 26 to be separated into an acceptable pulp and a primary reject material. The acceptable pulp from stage 26 which may be called a secondary acceptable pulp is combined or mixed with the acceptable pulp from separator stage 16 which may be called a primary acceptable pulp, and the mixed acceptable pulp may then go to a thickening stage 17 or other final stage. The primary reject material from separator 26 goes to a secondary and tertiary separator stage 27 where secondary and tertiary accepted lfractions are returned to drainer stage 21 for reprocessing with the coarse fraction from separator stage 20; alternatively the secondary and tertiary accepted fractions may be returned to drainer stage 23 for reprocessing with the less coarse fraction from stage 20. rlfhe reject material is discharged as waste, or it may be reclaimed as a by-product.

In the second stages of refining of the wood material, that is in the refining of the wood material not accepted by stages 14 and 16, there are separate renners for each fraction of this non-acceptable material. Because of this, different degrees and different types of refining can be given to each fraction (coarse and less coarse) as required. The treatment or processing may therefore be adapted to the material ybeing processed and the results required. The overall process is very versatile because of the changes that can ybe made in it. The amount of energy expended on the wood material by refiner stages 11, 22 and 24 may be varied and the fraction separation by stages 12, 14, 16, 2t?, 25, 26 and 27 may be varied. The process may be readily and conveniently adapted to produce pulps of different qualities for different products. In the case of ya coarser final pulp, for example, stages 16, 26 and 27 may be omitted. It will of course be apparent that the non-acceptable wood material from separator stage 16 may be divided into more than two fractions, and each of these fractions may be separately refined as required.

Referring now to FIGURE 2, there is shown a simplified partial side view, mainly in section, of a dise refiner 28 such as may be used in refiner stage 11 of FIGURE 1. The refiner 2S has a casing y30 enclosing a rotating disc 31 and a stationary disc 32. The rotating disc 31 cornprises Aa series of segmented plates 36 bolted or otherwise secured to a backing disc 33 which is supported on shaft 34. The shaft 34 is rotatably driven by a suitable motor (not shown). The rotational speed of the disc should be within the range 900 to 360() r.p.m. The central part 35 of rotating disc 31 is of a generally conical shape curving outwards towards the inner rim of plates 36. The portion 35 serves as a guide to direct wood chips from the central area outwards between discs 31 and 32.

The plates 36 have a surface that is sinuous in a radial direction. Reference to FIGURE 3 in conjunction with FIGURE 2 will illustrate one form of suitable plates 36. FIGURE 3 shows a plan View of part of two adjacent plates 36a and 36b. Radially extending ribs project yfrom the surface of plates 36 and these ribs are also sinuous or wavelike in a radial direction following the surface of the plates. In other Words, the plates 36 themselves and the ribs 47 both have troughs and crests at the same distances from the axis of the disc. The sinuous pattern extends lall the way around the disc 31.

The stationary disc 32 also comprises a series of segmented plates designated 37 which are bolted or other- Wise secured to a backing member 38. The plates 37 have a sinuous surface provided with ribs, but the plates 37 are designed to mate with plates 36 as shown in FIGURE 2. That is, there is a crest on plates 37 opposite a trough on plates 36. The above waveline plate pattern is of course only one example of a suitable form of plates. Depending on the physical shape of the raw chips and the speed of rotation of the disc other plate patterns and plate orientations may be required for optimum performance. For example, in place of the waveline profile, a sinuous path can still be provided by a fiat plate pattern similar to the pattern hereinafter recommended for refining stages 22 and 24. For higher speeds, higher pulp consistencies and flatter plate patterns it is desirable to arrange the disc and plates so that there is a tapered gap between the plates .36 and 37 so that the chips leaving the central part 35 will feed into the large end of the taper and progress outward along the sinuous path toward the narrow end of the taper between the plates. This taper may be straight or it may be slightly curved to give a gradually decreasing gap between the plates.

At the higher rotational speeds it may also be desirable to increase the coarser part of the plate area 36b, cornmonly referred to as the breaker-bar zone; here the coarser wood chips are broken up into finer fibrous fragments. For large machines, where the diameter of the rotating disc is for example greater than 45 inches, this can be more easily done without substantially limiting the finer plate area where much of the refining of the fibrous material is carried out.

As hereinbefore mentioned, the rotational speed of the disc should be in the 900 to 3600 rpm. range. While superior pulps have been made at the 900 r.p.m. level using moderately high consistencies in the range 642%, it has been found necessary to use higher rotational speeds ie. greater than 120:0 when operating at high pulp consistencies in the range -25 and higher. These higher speeds help to move the high consistency pulp through the sinuous path it must follow in order to effect the required flexing and fiberizing action. At the lower speeds, the high consistency pulp tends to choke the refiner to the point where the pulp will actually burn. In addition, of course, these higher speeds make it easier, from a purely mechanical and electrical point of view, to supply a high level of power to a given refining unit.

The refmer 28 may have a rotating control ring 40 made up of a plurality of segmented sections 40a, 401': (FiG-URE 3), secured to backing disc 33 adjacent the outer periphery of plates 36. While control ring 40 when it is used, may be secured directly to backing disc 33 which is part of rotating disc 3i, the control ring is usually considered to be a separate entity and is referred to separately. The control ring 40 may have its surface indented with a series of dimples 4S or may have another pattern incorporated in its surface. A stationary control ring 4l., which may be similar to control ring 40 in all respects, is secured to a ring mount 42. Ring mount 42 is adjustably mounted in frame 30 in a known manner for movement towards and away from ring 40. The spacing between facing surfaces of control rings 40 and 41 can thus be adjusted.

Not only is the control ring spacing adjustable, but the entire rotating disc 31 may be moved longitudinally towards and away from stationary disc 32 in a known manner to make the spacing between discs 31 and 32 adjustable. The adjustable spacing of both discs and control rings provides means to adjust the refining process for different pulp properties. The disc spacing is set for a required processing and the control ring spacing adjusted to give a desired rate of movement of wood material radially outwards between the discs. As a result, the length of time the wood material s retained between the discs can be controlled and the amount of energy expended on the wood material regulated.

There are, however, certain types of refineries which are designed to incorporate other novel features and such a design can make it difficult or impractical for such a reliner to be equipped with a control ring. Under such circumstances where a reflner not'equipped with a control ring has to be used, one may, by a careful selection of the various other embodiments of our invention make a pulp with properties similar to the present invention. One such selection, for example, could include proper use of tapered plates in conjunction with higher consistencies and higher rotational speeds.

It will, of course, be apparent that disc 32 in refiner 28 could be a rotating discalso. if both discs were rotating discs they would normally rotate in opposite directions.

f5 The refining process of the invention would not be affected by this.

The input to rener 28 comprises an input guide 43 which conducts wood chips fed into it to a screw conveyer 44 having a screw 45 rotated by a shaft 46. The output from refiner 28 is at the periphery of the discs where material is collected and discharged through discharge outlet 50.

The operation of reiiner stage 11 comprising refiner 28 will now be described. Wood chips are deposited in the input guide 43 where they are conducted to screw conveyor 44. The screw 45 of conveyor 44 moves the wood chips towards the central portion 3S of rotating disc 31 where the chips are guided into a path between the rotating disc 31 and the stationary disc 32. While the movement. of individual chips may be irregular during the time the chips are between the discs, the overall movement from input to output is a radial movement. As the wood chips move radially outwards along the sinuous path provided between plates 36 and 37 of discs 31 and 32, a mass of chips builds up between the plates. As the chips follow the sinuous path they are subject to continuously recurring flexing action which berizes the chips.

It is believed that the improved pulp derived from the lirst'stage refining in refiner 28 may be the result of the great amount of exing to which the chips are subjected. The flexing produces heat and the combination of the internally produced heat and the exing action may break the fibre to libre bonds and so iiberize the chips. The libre bonds are complex and involve cellulose to lignin bonds and lignin to lignin bonds. It is possible that the heat developed by ilexing may raise the temperature of the wood to the lignin softening point.

Thus, while the above describes how a superior mechanical pulp may be produced using a rotational disc speed, plate patterns, plate orientation and pulp consistency all within a certain range, in order to maximize this repeated compression-relaxation action which tends to fatigue the gross woody structure of the material and so release the strong flexible fibrous materials as well as to maximize the wood-to-wood contact, it is desirable to maintain the consistency of the pulp between the plates within the medium to high consistency range (ie. 5 to 45% based -on the oven-dry weight of the pulp). The actual choice of conditions will depend on a number of factors mainly economic in nature. In the case or production units requiring very high production rates, the tendency will be to choose high powered refining units (so as to minimize the number of units); this will require high rotational speeds and this in turn will suggest that the refiners be operated at as high a consistency as is feasible.

In one embodiment of the invention the first stage retiner (renner stage 11) may be substantially sealed from the atmosphere or pressurized. This will permit more of the heat generated by the flexing action and frictional action to be'retained within the refiner, and the refining action will take place in a region of higher temperature. No heat need be applied from an external source to achieve the higher operating temperature which is believed to aid in the breaking of the bre bonds.

It is desirable in rener 28 to keep to a minimum the shearing action between the metal surfaces of the plates and the wood chips. Excessive wood to metal shearing Contact abrades and tears the fibres and results in an excessive amount of undesirable chopped up pieces of wood instead of more desirable finer fibre aggregates. To help to keep the metal to wood shearing contact to a minimum and the wood to wood contact and flexing action at a maximum, it is desirable to use discs which have a sinuous or tortuous radial path and also to maintain the spacing between the rotating and stationary discs, or more specically between plates 36 and 37, greater than about 0.001 inch. The limits of acceptable disc spacing appears to be of the order of between 0.001 and 0.030 inch under most circumstances. The process, however, is very versatile and under some unusual conditions these limits could be exceeded, especially when operating at high consistencies. Normally, when operating in the medium consistency range of -15% the spacing will be of the order of between 0.005 and 0.020 inch with a preferred spacing of between 0.005 and 0.010 inch. When using consistencies higher than about say of the order of 15 to 25% or higher, the plate spacing becomes less critical and a wider range can be used without any ill effects. Generally, the spacing will lie within the wider range 0.020 to 0.100 inch.

As was previously mentioned, in order to increase pulp strength, increased amounts of power can be expended on the wood material in accordance with the process of this invention. To expend more energy in the rst stage of refining, that is in the reliner stage 11 of FIGURE l, the wood must be retained between the discs 31 and 32 (FIGURE 2) for prolonged periods. The passage of wood material radially between discs 31 and 32 may be controlled by changing the plate pattern and/or by changing the control ring spacing. Gbviously, if the depth of the waves in the surface of plates 36 and 37 is increased and the slopes of the waves made steeper, the passage of the chips or wood material will be restricted in a radial direction. It is also obvious that the passage will be restricted if the spacing between control rings 40 and 41 is reduced. A restrictive effect may also be obtained by choice of pattern on the control ring. For example, if wood is passing from a coarse sinuous plate pattern to a control ring having a fine flat pattern with a high surface resistance, it will encounter an abrupt change and its flow becomes restricted at the control ring even if the effective spacing between plates and control ring are the same. It is therefore possible to exercise very good control over the energy expended on the chips being processed.

It has been found that an expenditure of energy in the rst stage rener, that is in refiner stage 11 of FIGURE 1, of between about 80 and 200 I-IPD/ ADT (horse power days per air dry ton) will give a pulp with a high strength and good drainage properties from the rst stage alone. It should be noted that the aforementioned energy values refer to rener stage 11 alone and not to the overall process. Satisfactory and optimum values for the total process energy for Various qualities of pulp are discussed subsequently in connection with FIGURES 5a and 5b.

In ithe refiner stage 22, the disc refner may be similar to the disc refiner 28 of rener stage 11, except it should use the more conventional flatter-type plates having a regular barred pattern. The plate or disc spacing in rener stage 22 may be much less than that in rener stage 11 and the wood material is at a medium consistency (5 to 15%). In other words, refiner stage 22 may be operated in a conventional manner.

In the rener stage 24, the disc refiner may be similar to refiner 28 in refmer stage 11, and while it may use conventional-type plates, the preferred plate pattern is that where the surface is indente-d with a series of dimples. While refiner stages 22 and 24 may be similar to refiner stage 11 and operated in a similar manner the use of higher disc speeds together with the tapered orientation of the plates permits operation at high consistencies.

While metal plate surfaces have been more generally used for refner stages 11, 22 and 24, other materials having more abrasive surfaces have been tried (e.g., Alundum, Carborundum, etc.) and found very effective.

Referring now to FIGURE 4, there is shown a side View, partly in section, of a centrifugal separator or cleaner of the vortex pressure drop type. This type of centrifugal apparatus is known and has been used in the past for removing a small percentage of undesired particles from a pulp suspension. A description of an apparatus of this type and the manner in which it is operated to clean pulp may be found in United States Patent 2,927,693, of Freeman et al., issued Mar. 8, 1960. The apparatus is adapted in the process of this invention to separate a pulp suspension into two major fractions, and it may be conveniently used in separator stage 20. In separator stages 16, 26 and 27, it is used in a more conventional manner.

Briey, the separator which is designated 49 in FIGURE 4, comprises a barrel portion S1, a cone piece 52, a separation control enclosure 53, and a head piece 54. The head piece 54 has an inlet portion 55 which admits a suspension of wood material and introduces it tangentially into the top of barrel lportion 51, at passage S6, to cause a high velocity rvortex to occur in the barrel portion 51 travelling down to cone piece 52. Inwardly of passage 56 there is a cylindrical outlet aperture 57 extending lupwards to an outlet conduit 58. A first fraction of the wood material passes out the aperture 57 and conduit 53. The first fraction will comprise the lighter particles or fibres which are entrained in the more central upwardly moving portions of the vortex.

The heavier particles are thrown outwardly and settle down the walls into the control enclosure 53. Water or white water is introduced into enclosure 53 tangentially by control inlet 60 to create a vortex. This tends to dilute the downward vortex from cone piece 52 and causes heavier particles or bres to be thrown against the outer wall of the separation control enclosure 53 and to pass down these outer walls into a collecting ring 61 to discharge through outlet conduit 62. The second fraction comprising heavier pieces of material is thus discharged through conduit 62.

The separation may be controlled as is well known by varying the amount of water being injected through control inlet 60. As the fiow of water is increased through inlet 60 it tends to reverse some of the slower settling particles and entrain them in the inner upward moving vortex, while a decrease in water ow will allow more of the ne fibre to go out the outlet 62. This technique is known as elutriation and the enclosure 53 as the elutriator. Suitable selection of control water fiow and of the apparatus dimensions will adapt the apparatus to various consistencies, different wood material input suspensions, and different output fractions. In general, the elutriator is not used for separator stage 20 as the percentage rejection (i.e. the percentage of the second fraction) is very high. Because of its expense and because the use of an elutriator results in dilution of the stock, it is often only used in the last stage of centrifugal separation, such as, for example, in the tertiary separator of stage 27. If an elutriator is not required, then a centrifugal separator not having one is used. This type of separator is well known and is described also in aforementioned United States Patent 2,927,693.

It is believed that the following specific examples of processes in accordance with the invention will aid in providing an understanding of the invention. Reference is made in the examples to the drawings, and in particular to FIGURE 1.

Example I Chip bin 10 was provided with conventional spruce and balsam chips having a moisture content of t5-50%. The chips were fed into a disc refiner (reliner stage 11 of FIGURE 1) having a sinuous wave-like disc pattern and a dimple control ring pattern as shown in FIGURES 2 and 3. The spacing of the discs or plates was set at about 0.010 inch and the spacing of the control rings about 0.020 inch. While the control ring spacing seems to be such that it would exercise little restricting effect this is not the case. It should be noted that the :disc or plate spacing is measured from the closest points. With the sinuous pattern of the plates used this spacing would be measured from the tops of the ribs on the facing plates. The plate pattern is fairly coarse and the root of the ribs or the plate surface is considerably different from the top of the ribs. The plate surfaces are therefore, considerably farther apart than the ribs. The control ring spacing, on the other hand, would be measured from the surface of the ring. Consequently, with the spacings used, there is a definite restricting effect exercised on the flow of wood material by the control ring. A contributing factor to this restrictive effect would lbe the different patterns on the disc and ring as previously mentioned.

The refining consistency in the reiner of refiner stage 11 was about 8%, andthe temperature at the input about 50-600 C. with the output temperature about 70-90,o C. The energy expended in this stage was about 125 HPD/ ADT.

T'he output from refiner stage 11 was diluted to a consistency of about 2% and screened using 1/s inch perforated screen plates (coarse screen 12). The reject material from these screens represented about 10% of rener feed and was returned to the renner. The accepted portion from coarse screens 12, at a consistency of 1% was transferred to a horizontal centrifugal screen (fine screen 14) equipped with 0.060 inch perforated screen plates. The rejected material was directed to mixing tank 15, While the accepted material, at a consistency of about 0.50% Was passed through a centrifugal cleaner of the vortex pressure drop type (centrifugal separator 16). The inlet and outlet pressures respectively of the cleaner or separator were l p.s.i. and 1 p.s.i. The accepted pulp from the rst stage represented 50% of the total feed.

The non-acceptable wood material was then separated tertiary separator stage (secondary and tertiary separator stages 27). These separators in stage 27 may also be similar to those in stage 16. The accepted fractions from secondary and tertiary stage separators were returned to refiner stage 22 for reprocessing with the coarse fraction from separator stage 20. The rejected wood material from the tertiary separator in stage 27 was discharged as waste.

-In the preceding example, the process conditions were such that the first stage energy expen-der (rener stage 11) was i125 HPD/ADT. Similarly theI energy expended in the second stage refiners (stages 22 and 24) was 125 HPD/ADT and 85 HPD/ADT of the wood material being processed in each of the stages. The energies expended by stages 22 and 24 with respect to the total processed material was approximately 38 and 27 HPD/ADT, respectively. This gives a total process energy of about 125+38+27= 190 HPD/ADT This is considerably more energy than could be absorbed in a groundwood process for a pulp having the same freeness and it results in a much stronger pulp. -For purposes of comparison, various properties of pulp produced by the process conditions in the example, are given in Table I ywith comparable properties of pulps `obtained by two conventional processes.

TABLE I Process of this invention Process of coarse grinding and disc refining Property Refining Refining Benning Final Conventional Stage 1l Stage 22 Stage 24 Pulp Groundwood Pulp Energy 125 38 27 190 74 42 28 70 Freeness` 51 60 55 90 306 140 125 2. 39 2. 45 2. as 2. 5 3. e1 2. 54 2. 49 23.4 20.4 24 15 4.63 14.5 10.2 60 s1 75 45 4 5s 42 4,330 4,475 4,000 3,200 1,230 3.130 2, 350

75 24s 201 15 strength factorby a centrifugal separator of the vortex pressure drop Energy Horse power days per air dry-ton. type into equal fractions each representlng about 25% of Freeness Canadian standard freeness. the total feed. The inlet and outlet pressures of the separa- Bulk cc./gm. tor in separator stage 20 were respectively 35 p.s.i. and Burst factor TABPI Standard, 1 p sj, Tear factor yTAPfPI standard.

After passing through a lclrarner stage 21, the coarse 45 Breaking length Meters. fraction was processed by a disc renner (rener stage 22) Fold -Double M.I.T. at 0.5 kg. load. equipped with discs having a regular barred plate pat- Strength factor Burst +1/3 rear,

tern with a spacing of 0.005 inch and a control ring having a dimple pattern with a spacing of 0.010 inch. Tfhe consistency was approximately 7% and the temperature approximately C. The expended energy for the coarse fraction of wood material was about 125 HPD/ADT of the coarse fraction.

The less coarse fraction was passed through a drainer stage 23 and was then processed in `a disc rener (refiner stage 24) equipped with discs and a control ring both having a dimple pattern. The disc spacing was approximately zero and the control ring spacing 0.005 inch. The consistency was about 7% and the temperature 5060 C. About 85 HPD/ADT of energy was expended on the less coarse fraction. p

The pnl-ps from the second stage refiners (stages`22 and 24) were screened in a horizontal centrifugal screen using 0.060 inch perforated screens (fine screens 25). The rejected wood material was returned for reprocessing in refiner stage 22, and the accepted pulp was then processed through a primary centrifugal separator 26 under conditions similar to those -in centrifugal separator stage 16. The accepted pulp from separator stage 26 (secondary acceptable pulp) and the accepted pulp from separator stage 16 (primary acceptable pulp) are blended or combined and represent the total output of acceptable pulp. The primary rejected material from stage 26 was processed through a secondary separator stage and the secondary separator stage rejects were processed through a Table I indicates that it yis possible to expend greatel` amounts of energy in the process of this invention and thereby increase the strength of the resulting pulp.

Example Il Spruce and balsam chips were subjected t-o a similar process to that o-f Example I with the following modifications. Rener stage 11 was provided with a sinuous but flatter barred disc pattern similar to that previously used in retiner stage 22 and the plates were arranged to lea-ve a tapered gap. The spacing of the plates was 0.0120 inch at the closest point in the taper.

The consistency in retner stage 11 was about 25%. The temperature at the input 60 C. and the output temperature about C. The energy expended in stage 11 was about 80 HPD/ ADT. Similarly, the diatter plates, the tapered gap (no control ring) and a consistency of about 25% was used in refining the coarser fractions in reliner stages 22 and 24. The energy level in these latter stages was approximately 70 HPD/ADT giving a total pr-ocess energy of about HPD/ADT. While this energy is less than Example I, and consequently the strength of the final pulp was less than that shown in Table I for the previ-ous example, the properties of this pulp were still superior to that for conventional pulps. It will also be appreciated that the use of higher consistencies in the second stages will require more elaborate il and expensive thickening equipment than simple drainers (21 `and 23) (conventional pulp presses, for example, will serve at these stages). This may be avoi-ded by the use of high c-onsistencies and higher) in stage 11 and lower consistencies (545%) in stages 22 and 24.

When a medium pulp consistency is used, as in EX- ample `l, the process arrangement allows sufficient time for any tight, localized, high consistency wads or balls of pulp to become disentangled. However, when using high consistencies, as in Example 1-I, such tight balls of pulp are more prevalent and care must be taken to provide the proper -conditions for these wads of pulp to become `disen-tangled. While adequate dilution and time are important conditions, higher temperatures and mild agitation will accelerate the opening-up of these fibrous masses.

Referring now to -FIGURE 5a, the graph shows strength factor versus total process energy. Curve 64, shown as a solid line, indicates the general relationship between strength .and energy input for a pulp made by process in accordance with an embodiment of the invention, while cu-rve 65, shown as a dashed line, 4indicates the general relationship between strength and energy input for a conventional groundwood pulp. It is apparent from FIGURE 5a that the process of this invention permits increased amounts of energy to be expended -on the pulp resulting in improved strentgh properties. While the total process energy may vary within wide limits, Lfor example between 90 and 3'00 HPD/ADT, and achieve improved pulp, better results are achieved when the total process energy is between 120 and 200 HPD/ADT.

it may be seen that the curve in FIGURE 5a tends to level .of with increasing process energy. Consequently, it appears that with a conventional groundwood process, very little increase in strength is achieved by increasing the energy consumption past or 90 HPD/ADT.

FGURE 5b is a graph of freeness (Canadian standard freeness) versus total process energy. Curve 66, the solid line, indicates a general relationship between freeness and energy input for a pulp in accordance with the invention, while curve 67, the dashed line, indicates the sam-e relationship for a conventional groundwood process. It will be seen that the freeness of groundwood pulp tends to decrease more rapidly than the freeness of the pulp in accordance with this invention for the same increase in process energy.

Referring to FGURES 5a and 5b together, it will be seen that for a freeness of about 60, the respective energy inputs from curves 67 and 66 would be about 90 and 190 HPD/ADT respectively. The corresponding strength factors would be about 31 and 51 respectively. The strength factor for a pulp in accordance with this invention is of the order of 1.6 times that of conventional groundwood pulp. In addition, it appears in practice that the properties of groundwood pulp and pulp produced in accordance with the invention are sufficiently different that identical freeness measurements do not mean identical drainage characteristics of the pulps on a paper machine. The paper machine drainage characteristics of the pulp produced by this invention appear to be better than the freeness measurement (Canadian standard freeness) would indicate.

A further embodiment of this invention relates to the pressurization of at least one of the disc refiners used in the process of the invention. Because large amounts of energy are expended in the process, considerable amounts of heat are generated. If a disc refiner, such as refiner 28 of FIGURE 2, were pressurized, steam could be generated within the renner. This steam would be of the order of 50 p.s.i.g. and could be processed in a reboiler, if necessary, to provide cleaner steam for use in other paper machines. For example, the steam could be used in paper `machine driers. In addition the pressurization would enable the refining process to take place at a higher temperature to assist the fiberizing of the wood chips as was previously described.

While the invention primarily relates to a mechanical pulping process, it will be apparent that the advantages of the invention would still be achieved it chemicals were added at some point in the process or if chemically treated or partly cooked chips were used as the starting material. If chemicals are added during the process of the invention, the resulting pulp will be a semi-chemical pulp, but nevertheless the features of the inventive process will be used. Such semi-chemical processes are intended to fall within the scope of the invention. For example, it may be desirable to add a bleaching chemical to the wood material being processed. This may be desirable to brighten the pulp generally, or in the embodiment where steam under pressure is present in one of the refining stages and the refining is at an increased temperature it may be desirable to counteract a possible darkening efi'ect of the pressurized step. if bleaching chemicals are to be added, it is convenient to add them prior to a refining step, for example just before stage 11, so that chemical processing could take place concurrently with mechanical processing.

it will be seen that the present invention provides an improved proc-ess for making a mechanical pulp from wood chips.

We claim:

1. A process for the treatment of wood chips comprising the steps of:

disc refining wood chips,

separating the disc refined wood material into a primary acceptable pulp and a non-acceptable wood material,

separating the non-acceptable wood material into a coarse fraction and a less coarse fraction,

disc refining the coarse and less coarse fractions separately,

combining the disc refined coarse and less coarse fractions,

separating the combined fractions of wood material into a secondary acceptable pulp and a primary reject material, and

blending said primary and secondary acceptable pulps.

2. The process of claim 1 further comprising the steps of:

separating the primary reject material into =a secondary coarse fraction and a secondary reject fraction, and combining said secondary coarse fraction for additional treatment with said coarse fraction.

3. The process of claim l in which the separating of the primary acceptable pulp and the non-acceptable wood material is accomplished by screening and the remaining separating steps are `accomplished by centrifugal separating. j

4. The process of claim 1 wherein the total energy expended in disc refining is `between and 300 HPD/ADT of the total processed Wood material.

5. The process of claim 4 wherein said total amount of energy is between and 200 HPD/ADT.

6. The process of claim 1 wherein said wood chips are disc refined at a consistency in the range of 6 to 12%.

7. The process of claim 6 wherein said coarse and less coarse fractions are ydisc refined at a consistency of about 7% 8. The process of claim 1 wherein said Wood chips are Idisc refined at a consistency in the range of 15 to 25%.

9. The process of claim 8 wherein the coarse and less coarse fractions are disc refined at a consistency in range of about 5 to 15%.

v1f). A process for the treatment of wood chips oomprising the steps of:

disc refining Wood chips,

screening the disc refined wood material to separate it into a first non-acceptable wood material and an acceptable wood material,

centrifugally separating the acceptable wood material into a primary :acceptable pulp and a second nonacceptable Wood material,

mixing said `first and second non-acceptable Wood materials,

centrifugally 'separating Ythe mixed first and second nonacceptable Wood materials into a coarse fraction and a less coarse fraction,

Idisc refining the coarse fraction,

disc retining the less coarse fraction,

combi-ning the refined coarse and less coarse fractions,

screening the combined fractions of wood material to separate it into a third non-acceptable lwood material and an intermediate wood material,

centrifugally separating `the intermediate Wood material infto a secondary acceptable pulp and a primary reject material,

blending said primary and secondary acceptable pulps,

separating the primary reject material into a fourthnon-acceptable wood material land ya secondary reject wood material,

combining said fourth non-acceptable Wood material with said third non-acceptable wood material for lreprocessing with said coarse fraction, an-d discharging said secondary reject Wood material.

References Cited UNITED STATES PATENTS 2,175,484 1O/19139 Rees et al. l241---24 XR 2,870,908 l/ 19'59 iFitch 24d-24 XR 2,947,655 8/ 1960 Eberhardt 241-24 XR 3,327,952 `6/19671 Rosenfeld 241-28 XR WILLIAM W. DYER, JR., Primary Examiner. W. D. BRAY, Assistant Examiner. 

1. A PROCESS FOR THE TREATMENT OF WOOD CHIPS COMPRISING THE STEPS OF : DISC REFINING WOOD CHIPS, SEPARATING THE DISC REFINED WOOD MATERIAL INTO A PRIMARY ACCEPTABLE PULP AND A NON-ACCEPTABLE WOOD MATERIAL, SEPARATING THE NON-ACCEPTABLE WOOD MATERIAL INTO A COARSE FRACTION AND A LESS COARSE FRACTION, DISC REFINING THE COARSE AND LESS COARSE FRACTIONS SEPARATELY, COMBINING THE DISC REFINED COARSE AND LESS COARSE FRACTIONS, SEPARATING THE COMBINED FRACTIONS OF WOOD MATERIAL INTO A SECONDARY ACCEPTABLE PULP AND A PRIMARY REJECT MATERIAL, AND BLENDING SAID PRIMARY AND SECONDARY ACCEPTABLE PULPS. 